Astronomy and Astrophysics – Astronomy
Scientific paper
May 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000aas...196.2802m&link_type=abstract
American Astronomical Society, 196th AAS Meeting, #28.02; Bulletin of the American Astronomical Society, Vol. 32, p.715
Astronomy and Astrophysics
Astronomy
Scientific paper
We have used the Wisconsin Hα Mapper (WHAM) to observe the spatially extended distribution of ionized hydrogen in M31 beyond the stellar disk. We obtained five sets of observations, centered near the photometric major axis of M31, that extend from the center of M31 to just off the edge of the observed southeastern HI disk. We find an Hα intensity of IHα ≈ 0.12R beyond the bright stellar disk but within the HI disk (1R = 106 / 4π photons cm-2 s-1 ster-1 = 2.41 x 10-7 erg cm-2 s-1 ster-1). Just within and off of the outer HI disk, we find no significant detection of Hα , and place an upper limit on the intensity equal to our sensitivity limit of IHα < ~ 0.04R. Our sensitivity limit is dominated by the systematic uncertainty in the shape of the spectral baseline generated by weak spatially and temporally varying atmospheric emission lines. Although a 2σ uncertainty due to Poisson noise is at the ≈ 0.008R level, an emission line with an intensity up to ≈ 0.04R may be present due to systematic errors associated with the subtraction of the atmospheric lines. Our results show a gradient in IHα within the HI disk out to radii well beyond the stellar disk. This indicates that any uniform source of ionization, such as the intergalactic Lyman continuum flux Φ 0, produces less than ≈ 0.04R of Hα radiation. If we assume that the outer HI disk aborbs all incident Lyman continuum photons, that this disk is optically thin to Hα photons, and use the geometry of the inclined disk, an IHα < ~ 0.04R implies that Φ 0 < ~ 1.0 x 104 photons cm-2 s-1. This upper limit is well below the required photoionization flux incident upon high-velocity clouds in the Milky Way's halo (Tufte et al. 1998) and in the Magellanic Stream (Weiner & Williams 1996). Therefore an additional source of ionization, such as the stellar disk of the Milky Way, is present in the Galactic halo (Bland-Hawthorn & Maloney 1999). This work was supported by the NSF through grant AST96-19424.
Haffner Matthew L.
Madsen Greg J.
Maloney Philip R.
Reynolds Ron J.
Tufte Stephen Louis
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